Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to drive circuits for voltage-controlled switches and, more particularly, to drive circuits which provide protection of the voltage-controlled switches from excessive voltages at their conduction control terminals.
2. Description of the Background
A voltage-controlled switch is controlled by applying a voltage to its conduction control terminal (called the gate for a field effect transistor). Practical voltage-controlled switches, such as metal-oxide-semiconductor field effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs), pose limits to the control voltage applied to the conduction control terminal. Even short-duration voltage levels beyond these limits may lead to reliability problems or destruction of the switch.
Quite frequently, gate drive voltage levels delivered by gate drive circuits are not well controlled, and can vary over a relatively wide range. If this range extends beyond the maximum gate levels of the switch, a gate voltage protection circuit is required.
A prior art gate voltage clamping circuit is shown in FIG. 1. Bipolar voltage clamping is achieved using Zener diodes 10, 12. If the voltage level of the drive voltage reaches the Zener voltage of the diodes 10, 12, both diodes 10, 12 start conducting, thus protecting the switch 14 from excessive gate voltage stress. Because of the xe2x80x9cback-to-backxe2x80x9d connection of the diodes 10, 12, one of the diodes 10 operates in forward mode, and the other diode 12 operates in avalanche mode during the clamping action. The drawback of this protection scheme is the power dissipation in the clamping diodes 10, 12, particularly if the output impedance of the gate drive circuit is low and/or the maximum unclamped voltage is high. Moreover, a low output impedance of the gate drive circuit is essential for high-speed switching of the switch 14.
A prior art unipolar drive circuit is shown in FIG. 2. A bipolar junction transistor (BJT) 16 is connected in an emitter-follower configuration. The base voltage of the BJT 16 is clamped to a defined level using a Zener diode 18 and a resistor 20. The maximum voltage applied to the gate of the switch 14 (with respect to its source) is approximately the Zener voltage level of the diode 18 minus the base-emitter junction voltage drop (Vbe) of the BJT 16. Because the circuit provides no discharge path, an anti-parallel diode 22 is required to allow the gate drive circuit to discharge the gate of the switch 14. The primary drawback of this drive circuit is its relative complexity and the poor turn-on performance. The base current of the BJT 16 is limited by the resistor 20, which is required to control the power dissipation in the Zener diode 18. Another drawback of the circuit is that the voltage at the gate of the switch 14 is always reduced by one Vbe voltage drop, even if the drive voltage is low anyway. Steady-state power dissipation of the scheme can cause further problems.
Another prior art unipolar drive circuit is shown in FIG. 3. Enhancement MOSFET 24 is connected in a source-follower configuration. The gate of enhancement MOSFET 24 is positively biased using a voltage source 26. When a positive drive voltage (Vdrive) is applied, the gate of the switch 14 follows this voltage up to a level equal to the bias voltage 26 minus the gate-source threshold voltage (Vgsthres) of the MOSFET 24. This circuit has several advantages. Neglecting the voltage source 26, complexity of the circuit is low. If an adequately sized enhancement MOSFET 24 is used, the turn-on drive impedance can be made very low. The circuit does not suffer from steady-state power dissipation. Even if a high drive voltage is supplied continuously, the enhancement MOSFET 24 is in cutoff mode, and no significant current is drawn. The disadvantage of the scheme is the necessity of the bias voltage source 26. Moreover, if a bias voltage source with a suitable voltage level is not available, the complexity of the circuit increases significantly.
Accordingly, there exists a need for an efficient, simple drive circuit for a voltage-controlled switch that has a low output impedance and low steady-state power dissipation.
The present invention is directed to a drive circuit for a voltage-controlled switch. According to one embodiment, the drive circuit includes a normally-on switch including first and second terminals and a control terminal, wherein, the first and second terminals have a conduction path therebetween, the second terminal is connected to a conduction control terminal of the voltage-controlled switch, and the control terminal of the normally-on switch is biased by a drive voltage relative to the first terminal of the normally-on switch. The normally-on switch may be, for example, a depletion mode MOSFET. The drive circuits of the present invention may be implemented in, for example, power converter circuits.
The present invention represents an advantage over prior art mechanisms for protecting the conduction control terminal of a voltage-controlled switch from excessive voltages because of its reduced complexity and efficiency. The present invention offers a further advantage of having a low output impedance and low steady-state power dissipation. These and other benefits of the present invention will be apparent from the detailed description hereinbelow.